The present invention relates to a directional microphone apparatus employing an array of microphone units to provide sharp directivity, and in particular to a directional microphone apparatus of this type whereby the range of sharp directivity extends down to relatively low frequencies within the audio range.
FIG. 1 illustrates an example of a prior art type of directional microphone apparatus in which an array of microphone units M1 to M9 are arranged in line to provide sharp directivity along the direction indicated as 0.degree.. Such a microphone apparatus provides sharp directivity within a specific plane, e.g. the horizontal plane, in which the microphone units are linearly arrayed, with directivity in the vertical direction being determined by the directivity response of the individual microphone units. The microphone units M1 to M9 are unidirectional units, whose output signals are summed by an addition circuit ADD to produce an output signal which is applied to an output terminal 1. The frequency response of the output signal from such a prior art directional microphone apparatus is of the form shown in FIG. 2, with the curve indicated as "0.degree." being the characteristic for sound reception in the 0.degree. direction indicated in FIG. 1, and the curve indicated as "90.degree." being that for reception in the 90.degree. direction. FIG. 3a shows the directional response characteristic of the directional microphone apparatus of FIG. 1 at the low frequency indicated as .alpha. in FIG. 2, while FIG. 3b shows the corresponding response at the high frequency indicated as .beta. in FIG. 2. It can thus be understood that the prior art apparatus of FIG. 1 provides sharp directivity within a range of relatively high frequencies, but provides very much less sharp directivity within a lower range of frequencies. This is due to the fact that the directivity of the microphone unit array at low frequencies becomes determined by the directivity of each of the individual microphone units. This difference in directivity between these high frequency and low frequency ranges presents various problems, e.g. with respect to attaining high fidelity sound reproduction, pick-up of sounds from unwanted sources, etc.
FIG. 4 shows an example of a prior art type of directional microphone apparatus which has been proposed for overcoming the problem described above. In FIG. 4, Ma1 to Ma5 respectively denote unidirectional microphone units, while Mb1 to Mb4 denote omnidirectional microphone units. The output signals from the unidirectional microphone units Ma1 to Ma5 are summed by an addition circuit ADD.sub.a, while the output signal from the addition circuit ADD.sub.a is inputted to a subtraction circuit SUB.sub.a. Of the output signals from the omnidirectional microphone units Mb1 to Mb4, the output signals from Mb2 and Mb3 are applied to respective addition inputs of an addition/subtraction circuit ASC.sub.a, while the output signals from the units Mb1 and Mb4 are applied to respective subtraction inputs of addition/subtraction circuit ASC.sub.a. An output signal is thereby produced from the addition/subtraction circuit ASC.sub.a which represents the second order bidirectional sound pressure gradient characteristic of the microphone unit array. This signal is inputed to an equalizer circuit EQ, and the resultant equalized output signal is applied to a subtraction input of subtraction circuit SUB.sub.a, to be subtracted therein from the output signal from addition circuit ADD.sub.a.
The frequency components of the output signal from addition/subtraction circuit ASC.sub.a in the low-frequency range, for the 90.degree. reception direction, will be almost identical to the corresponding low-frequency components of the output signal from ADD.sub.a. As a result, the output signal from SUB.sub.a resulting from subtraction of the equalized output signal from ASC.sub.a from the output signal of ADD.sub.a will have improved directivity in the low-frequency range. The frequency response of this output signal from is of the form shown in FIG. 5, with the curves designated "0.degree." and "90.degree." corresponding to those of FIG. 2 described above. FIG. 6a shows the directional response characteristic of the output signal from SUB.sub.a at the low frequency indicated as .alpha. in FIG. 5, while FIG. 6b shows the corresponding response at the high frequency indicated as .beta. in FIG. 5. It can thus be understood that directivity in the low-frequency range is provided by the prior art directional microphone apparatus example of FIG. 4, by comparison with that of FIG. 1. However the apparatus of FIG. 4 has the basic disadvantage that it is necessary to provided additional microphone units by comparison with the number of microphone units required in the directional microphone apparatus of FIG. 1 (i.e. to attain comparable directivity in the high-frequency range), these additional microphone units being omnidirectional. This results in substantially increased manuacturing cost. In addition, the incorporation of these additional microphone units into the microphone unit array will result in a reduction of the signal/noise ratio of the output signal from the directional microphone apparatus. In order to compensate for this lowering of the signal/noise ratio, it would be necessary to incorporate additional unidirectional and omnidirectional microphone units into the microphone unit array, which again will result in increased manufacturing cost.